The use of anti-oxidants in the polymer industry is of major importance because they extend the polymer's useful temperature range and service life. Anti-oxidants are substances which when added in small proportions to the polymer, retard its degradation due to oxidation at ambient and elevated temperatures by opposing or inhibiting reactions promoted by oxygen or peroxide. In finished polymer products, protection against oxidative deterioration is required in order to maintain the original properties of the composition. In selecting a suitable anti-oxidant for a polymer system, various factors must be considered such as polymer structure, impurities in the polymer such as metal contaminants from raw materials or the catalyst used to produce the polymer, the end use of the polymer, and finally, the presence of fillers and various ancillary compounding materials.
Oxidation of polymers is normally caused by the internal formation of free radicals and since hydroperoxides are usually the most important source of these free radicals, it is desirable to decompose the hydroperoxide free radicals to nonradical products and thus destroy or inhibit the oxidation process. Many compounds have been found to inhibit oxidation by destroying organic peroxides or hydroperoxides, among which are the mercaptans, sulfides, sulfones, phenols and salts of dithiocarbamic and dithiophosphoric acids.
It has also been known in the past to employ two or more anti-oxidants simultaneously in a polymer in order to achieve a synergistic effect, i.e., the combined effect of the two anti-oxidants will be greater than that of either one at a level equal to their concentration in the polymer. As an example, in the stabilization of polyolefins, particularly polypropylene, a synergistic anti-oxidant action occurs between phenols such as 2,6-di-tertiary-butyl-4-methylphenol and sulfides such as dilauryl-3,3'-dithiodipropionate in which the sulfide acts as a peroxide decomposer and the phenol acts as a free radical interceptor.
The prior art is replete with patents teaching combinations of anti-oxidant materials. Examples include Kuckro et al. (U.S. Pat. No. 3,819,410) which teach the combination of a hindered phenol and a zinc dialkyl dithiophosphate, Holt (U.S. Pat. No. 3,655,559) which teaches the combination of a hindered phenol, alkylated aromatic amine, dithiophosphite and a metal salt of a chelating agent. Kauder (U.S. Pat. No. 3,458,472) teaches stabilizing polyvinyl chloride with a cyclic crystalline organo tin compound, hindered phenol, aromatic amine, alkaline earth metal stearate and mercapto acid salt. Brnachesi, (U.S. Pat. No. 3,622,530), Stretanski (U.S. Pat. No. 3,763,066), Koenen et al. (U.S. Pat. No. 3,779,962), Anderson et al. (U.S. Pat. No. 3,755,450) and Jennings et al. (U.S. Pat. No. 3,764,571) teach the use of various stearates in anti-oxidant combinations. Sommerville et al. (U.S. Pat. No. 2,802,811) teaches the use of the zinc salt of 2-mercaptobenzothiazole as an anti-oxidant.
It is the object of this invention to provide a new synergistic combination of anti-oxidants which greatly improves the heat stability of peroxide cured vulcanizates of unfilled low density polyethylene or mineral-filled ethylene-vinyl acetate copolymer compositions without any undesirable staining of the composition. This and other objects of the invention will become apparent to those skilled in the art from the following detailed description.